
"Mathematical thinking is not the same as 'doing math'--unless you are a professional mathematician. For most people, 'doing math' means the application of procedures and symbolic manipulations. Mathematical thinking, in contrast, is what the name reflects, a way of thinking about things in the world that humans have developed over three thousand years. It does not have to be about mathematics at all, which means that many people can benefit from learning this powerful way of thinking, not just mathematicians and scientists."--Back cover.
Now, What do You mean to "learn Math". You have to be more specific with your goal. This is funny coming from me because I haven't able to specialize in any single topic.
Let's suppose You somehow saw The Beauty of Math somewhere and You are ready to continue your journey in such a beautiful topic.
I really believe You Will enjoy reading "Mathematics for The million" by Lancelot, I saw this recommended by Sir Anthony Hoare[2], You Will be amazed by its content (I have only ready chapter one).
What's Mathematics?
Are elementary school children doing Math when they solve addition and subtraction problems?
Is The High school student crunching mechanical procedures, plugging numbers into formulas doing Math?
Is theorem proving Math?
Daphne Koller said in an interview that we are all doing probabilistics when Thinking and making certain decisiones[3]. Are we all Mathematicians?
Do You want to study Math as a tool to help You in some other areas or study it itself as a subject?
It doesn't matter, You Will enjoy it and love it.
Trying yo answer your question. Learn some Arithmetic, just enough. Crunch some elementary algebra (in The fashion of Visualizing Algebra provided by Udacity). Read The book Geometry by Tatiana Alekseyevskaya, learn some Discrete Math (I recommend You the book on the topic by Thomas VanDrunnen), read the other four books in the gelfand series. Learn some logic. Read Introduction to Mathematical Thinking by Keith Devlin. I think at this point You Will able to read some resources o calculus or abstract Algebra.
If this seems like a Lot, don't dismay. There was this guy George Green having to take care of mill to make bread and do also found time for Mathematics, apparently DeMoivre was struggling financially. There people are just like You. Enjoy your Math journey
[1] The interview is available on Lex Fridman YouTube channel.
[2] The conversation between him and Leslie Lamport is available on YouTube
Introduction to Mathematical Thinking by Keith Devlin (Stanford) https://www.coursera.org/learn/mathematical-thinking
I did the "Introduction to Mathematical Thinking" MOOC from Keith Devlin. The curriculum is available as a book as well.
The class is basically how to write and read proofs for non-math majors. It starts pretty slow, but gets harder at some point. The number theory proofs were fun.
You 'got to' grade others proofs online, and they graded yours which was an interesting way to get familiar with reading and writing proofs.
I recommend it because instead of an area of math it focuses on what it means to prove something. And the teacher is pretty entertaining.
https://www.amazon.ca/Introduction-Mathematical-Thinking-Kei...
In the spirit of OP's question:
How to Solve it by G. Polya
Solving Mathematical Problems by Terrence Tao
Introduction to Mathematical Thinking by Keith Devlin
Are all amazing, How to Solve it in particular is an all time classic.
https://www.coursera.org/learn/mathematical-thinking?
This brought me what math is about:)